The present invention relates generally to the formation of and application of increased-current gas-cluster ion beams (GCIB's) for processing the surfaces of workpieces, and, more particularly to reducing space charge effects in GCIB's, reducing workpiece charging, and to improving the measurement accuracy of GCIB currents and doses.
The use of GCIB's for processing surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi et al.) in the art. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters typically consist of aggregates of from a few to several thousand molecules loosely bound to form the cluster. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of controllable energy. Such ions typically carry positive charges of q×e (where e is the electronic charge and q is an integer greater than or equal to one). The larger-sized clusters are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ion clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of conventional monomer ion beam processing.
Means for creation of and acceleration of such GCIBs are described in the reference (U.S. Pat. No. 5,814,194) previously cited. Presently available ion cluster sources produce clusters ions having a wide distribution of sizes, N (where N=the number of molecules in each cluster—in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as a molecule and an ionized atom of such a monatomic gas will be referred to as a molecular ion—or simply a monomer ion—throughout this discussion).
Many useful surface processing effects can be achieved by bombarding surfaces with GCIB's. These processing effects include, but are not necessarily limited to, smoothing, etching, and film growth. In many cases it is found that in order to achieve industrially practical throughputs in such processes, GCIB currents on the order of hundreds to thousands of microamps are required. Space charge effects in ion beams are expected when Poissance as defined by A. T. Forrester in Large Ion Beams, Wiley, New York (1987) approaches unity. In the case of a 400 μA beam with an N/q ratio of 5000, the Poissance varies with the GCIB acceleration voltage up to about 0.3 or so depending on exact operating conditions. Accordingly, some space charge beam expansion would be expected and is observed. Particularly at low acceleration voltages, providing a degree of space charge neutralization to the beam by providing a source of low energy electrons enhances the ability to transport larger gas-cluster ion beam currents and reduces beam spot size. In the beamline of a practical production GCIB processing tool, to minimize beam loss due to space charge expansion of the beam, it is useful to keep the beamline as short as practical (˜50 cm) and the beam size at the workpiece is nevertheless relatively large (˜6 cm). In the prior art, a simple thermionic electron emitter in the vicinity of the beam has been used to provide space charge neutralizing electrons. In order to achieve successful transport of higher beam currents (compared to the typical hundred or so microamps in practical prior art GCIB tools) it is highly desirable to achieve more effective space charge neutralization in the GCIB.
Another important consideration in extending the useful GCIB beam currents to increase processing throughput is the fact that the workpiece can be charged up by the effects of the GCIB bombardment. This is especially important when the workpieces are semiconductor substrates, magnetic memory sensors, or other charge sensitive materials. Workpiece surface charge neutralization is required for successful GCIB processing. In some applications, such as magnetic memory smoothing, the requirements are even more stringent than for semiconductor devices and a maximum surface charging of ±6 volts or even less is required for successful processing. Low energy electrons supplied to the GCIB and the workpiece surface can provide surface charging control as well as GCIB space charge control, but in order to achieve low workpiece charging potentials under varying conditions, such electrons must be low energy. In the past it has not been practical to achieve satisfactory space charge neutralization and to simultaneously control workpiece surface charging at acceptably low potentials. Simple thermionic filament electron sources act as space-charge-limited-diodes, and thus do not readily emit adequate quantities of electrons. It is possible to dramatically reduce the space-charge-limited-diode effect by using an accelerating potential to extract electrons from a thermionic filament's space charge region. This can dramatically increase electron current emission, but results in an increased electron energy problem and in unacceptable risk of workpiece negative charging by energetic electrons if the GCIB should fluctuate momentarily or be momentarily interrupted. Thus, while an accelerated electron source can provide suitably high electron currents, the risk of high energy electrons charging the workpiece make the method unacceptable in many sensitive applications.
For GCIB process control purposes, it is important to be able to measure and control the GCIB intensity. One convenient way of achieving this is by measuring the GCIB current. Faraday cups have traditionally been used as ion beam current measuring devices and are well known in the art of conventional monomer ion beams and have been used successfully for low current GCIB measurement. Inherently, a gas-cluster ion beam transports gas. For an argon beam having a beam current, IB, the gas flow, F (SCCM), in the beam is
                    F        =                  2.23          ×                      10                          -              18                                ⁢                      (                          N              q                        )                    ⁢                                          ⁢                      (                                          I                B                            e                        )                                              (                  Eqn          .                                          ⁢          1                )            
With a beam current of 400 μA and an N/q ratio of 5000, the beam conducts a gas flow of 27 SCCM. In a typical GCIB processing tool the ionizer and the workpiece being processed are each typically contained in separate chambers. This provides for better control of the substrate processing pressure. However, a major area of difficulty with beams carrying large amounts of gas occurs in terms of beam current measurement. The entire gas load is released when the cluster beam strikes the inside of the faraday cup. Charge exchange and gas ionization by the beam within the confines of the faraday cup become extreme and significant measurement errors occur with conventional faraday cup designs.
It is therefore an object of this invention to provide a neutralizer capable of providing large neutralizing electron currents but having low electron energy.
It is also an object of this invention to provide a method of effective space charge neutralization of a high current GCIB.
It is a further object of this invention to provide an improved method of limiting the charging of the surface of a workpiece being processed by GCIB.
Another object of this invention is to provide an improved faraday cup for beam current measurement in beams having high gas transport.
A still further object of this invention is to provide a method for accurate measurement of gas-cluster ion beam current in GCIBs transporting large amounts of gas.